Alkali metal halide-assisted chemical vapor deposition (CVD) methods can produce wafer-scale uniform monolayer transition metal dichalcogenides (TMDs). Further defect engineering is necessary to obtain high-performance functional devices. While defect engineering has focused on the surface of the monolayer TMDs or the contact property, interface defect engineering is rare and non-trivial. Based on a NaCl-assisted CVD-grown large-scale uniform MoS2 monolayer on SiO2/Si substrate, a trace amount of Na cations is present, residing at the SiO2 substrate during the CVD-growth process and contributes to the n-type doping into the supported monolayer MoS2. Furthermore, the residual Na cations are electrically moved toward the bottom side of monolayer MoS2 to passivate the interfacial defects.